1. Field of the Invention
The present invention relates to a phase shift mask, and more particularly to a half-tone phase shift mask capable of preventing light from being transmitted in undesired areas, and thus capable of obtaining desired fine patterns.
2. Description of the prior Art
With the development of the highly integrated circuits, such as highly integrated dynamic random access memory devices, the demand for fine patterns is on the increase. In an effort to reduce the pattern size, a half-tone phase shift mask has been developed.
Generally, a half-tone phase shift mask is comprised of a quartz substrate and mask patterns including two layers, a light screen material layer and a phase shift material layer. In the half-tone phase shift mask, the light screen material layer is made of a chrome layer which transmits an incident light to a predetermined transmissivity. The phase shift material layer formed on the light screen material layer plays the role of shifting the phase of light at an angle of 180. Such a phase shift mask is designed to keep constant the amplitude of light illuminated on a wafer in the light exposing process and to minimize the light intensity caused by the interference between the light passing through the phase shift material layer and the light passing through the adjacent quartz substrate, thereby improving the resolution of the photosensitive film patterns.
FIG. 1 is a cross sectional view of photosensitive film patterns with a conventional half-tone phase shift mask. As shown in FIG. 1, positive photosensitive film patterns 15 formed on a semiconductor substrate 14 are shown with a conventional half-tone phase shift mask and the path of the incident light 16. A transparent substrate 11 has mask patterns which are made of two material layers, a chrome layer 12 and a phase sifting material layer 13. The chrome layer 12 has such a thickness so as to provide a predetermined transmissivity.
The area, which is not intended to be exposed, may be illuminated by the exposing light transmitting the mask patterns formed on the mask substrate. This causes the formation of an uneven surface on the top of the photosensitive film patterns 15. The size of the uneven surface on the photosensitive film patterns increase in proportion to the size of the mask patterns and decreases in proportion to the distance between the patterns.
Such photosensitive film patterns having an uneven surface decrease the reliability of the semiconductor devices and their yield. For example, when a photosensitive film formed by a conventional half-tone phase shift mask is used as an etching mask or as an ion implanting mask, a layer under the mask may be damaged or ions may be implanted into unintended areas.
In particular, in the process of exposing the positive photosensitive film coated on a semiconductor substrate using such a conventional half-tone phase shift mask, the area which is not intended to be exposed may become illuminated because of the transmissivity of the light passing through the mask patterns. The incident light passes through the mask patterns in the unintended areas and the intensity of the passing light is not uniform over the unintended areas. As a result, photosensitive film patterns having uneven surfaces are formed. These photosensitive film patterns have the desired height only on the edge thereof. However, the center of the photosensitive film in the area, which is not intended to be exposed, is illuminated by the passing light having weak intensity and variable kinds of intensity. Accordingly, in the process of developing, the center portion of the photosensitive film in the unintended area may be removed at different thicknesses according to the size thereof.
When light is illuminated on such a half-tone phase shift mask, the intensity of light is maximized at the center portion of a transmission region, which has no mask pattern, and the light intensity is reduced to zero at the edge of the transmission region. As stated above, the light intensity peak at the light screen areas provided with mask patterns may be detected, although it is very weak. The peak of light intensity detected in the light screen area is called a side-lobe peak.